1. Field of the Invention
The invention relates to a phase-locked loop comprising a phase detector and an oscillator controlled by means of a switching network.
2. Description of the Related Art
Phase-locked loops of this type are necessary, for example, for clock supply arrangements in digital data and transmission networks. The phase-locked loop in essence comprises a voltage-controlled oscillator, a loop filter and a phase detector which compares the phase position of a signal produced by the voltage-controlled oscillator to the phase position of an input signal. The control voltage produced by the phase detector readjusts the oscillator via the loop filter until the phase difference is minimized or has reached a preset nominal value.
In German Offenlegungsschrift DE-A 33 24 919 a phase-locked loop is disclosed comprising a voltage-controlled oscillator, whose output is connected to a first input of a phase detector. At the second input of the phase detector a signal having a reference frequency is available. Between the phase detector and the voltage-controlled oscillator a counter is inserted as well as a switching network comprising switching elements, which is for example arranged as a capacitance network and then comprises as switching elements capacitors having different capacitances. The frequency of the voltage-controlled oscillator can be adjusted in steps via the switching network by shunting various capacitors. Consequently, the accuracy with which this frequency can be adjusted is restricted by the capacitance tolerances of the capacitors in the switching network, specifically if the reference frequency drops out. Another voltage-controlled oscillator using a switchable capacitor network for frequency control is shown in UK Patent Application No. GB 2 120 478A.